This invention pertains generally to acoustic positioning systems and more particularly to reactor vessel in service inspection manipulator positioning systems.
Commercial power nuclear reactors include a vessel which is generally a cylindrical metallic member having a top flange welded to it, with a plurality of nozzles attached to and extending through the vessel wall. Numerous welds are used in fabricating the vessel, in joining the flange to the cylindrical portion of the vessel, as well as in providing the inlet and outlet nozzles. While the reactor vessel is itself encased in a thick concrete containment area in the plant, the structural integrity of the reactor vessel must be assured.
The weld areas of the reactor vessel are inspected before the vessel is placed in use, and in-service inspection of the vessel weld areas is desirable and required by Governmental regulation. Such in-service inspections are usually carried out in an underwater, radioactive environment with remote control operation, while maintaining a high degree of precision of placement and movement of the inspection tooling.
As a practical matter, the tooling designed for this purpose must have the capability of accommodating a variety of sizes of reactor vessels.
A number of in-service inspection manipulators exist in the art, such as the manipulator taught in U.S. Pat. No. 3,809,607, to T. R. Murray et. al., issued May 7, 1974. Manipulators of this type generally include fixturing adapted to be keyed on a predetermined location on the reactor vessel. The fixturing is generally employed to support the manipulator and serve as a reference for a number of encoders which track the location of inspection transducers so that a complete and coordinated map of the vessel and its appendages can be made. Normally, ultrasonic transducers are employed to perform the nondestructive inspection mapping. The transducers are generally supported, cantilevered off of a central column as described in the above noted patent. The inspection transducers' outputs with respect to their position in the vessel are recorded as the transducers are scanned over the required surface of the vessel and its appendages to satisfy the volumetric examination required by Governmental regulation. This record is then evaluated and compared with previous mappings to determine the state of the vessel.
It can be appreciated therefore, that in order for any volumetric examination to have value it must be made to exactly correspond to previous mappings. This requires that the transducers be moved in a manner to replicate previous examinations. Therefore, exact positioning and scanning of the transducers is not only desirable, but necessary to the success of the examination. Any improvement in accomplishing replication of the scanning process therefore significantly enhances the reliability of the result that is achieved. Normally, inspection manipulators of this type have a number of movable joints each having an encoder which provides an output representative of the relative positioning of its corresponding connecting elements or limbs. The combination of the encoder's outputs together with the rigidity of the manipulator generally assures the accuracy of positioning the transducers around the reactor vessel. However, when the manipulator's various joints are extended in the course of an in-service inspection in an irradiated reactor vessel environment, the exact mechanical positioning of the inspection transducers by the manipulator as reflected by the encoder outputs may vary slightly from the transducers' true position with respect to the vessel.
Therefore, to assure the accuracy and exact duplication of each scan performed during a volumetric examination, it is desirable to have an independent position indication system which can assure the exact location of the transducer extension of the manipulator arm within the reactor vessel cavity. More particularly, it is desirable to have such a position indication and location device for exactingly positioning the scanning transducers within the apertures of the reactor vessel nozzles.